OneWayToSource in Silverlight and UpdateSource issue

I'm facing some problems binding a CommandParameter to its own Command in an application built using Prism 2.2 as MVVM . Let me introduce what it's happening. I've got a customized listbox with a property named NumPageElements, and a couple of buttons to scroll through the list who needs that property. A simplified xaml of what I need (and works) in wpf is:

<Button x:Name="PageDownButton" Command="{Binding PageDownCommand}" CommandParameter="{Binding ElementName=ItemsListBox, Path=NumPageElements}" />

<Custom:MyOwnListBox x:Name="ItemsListBox" x:NumPageElements="{Binding ElementsPerPage, Mode=OneWayToSource}" >
. . .
</Custom:MyOwnListBox>

To have the same behaviour in开发者_高级运维 Silverlight I wrote this xaml:

<Button Name="PageDownButton" Command="{Binding PageDownCommand}" CommandParameter="{Binding ElementName=ItemsListBox, Path=NumPageElements}" />

<Custom:MyOwnListBox Name="ItemsListBox" NumPageElements="{Binding Path=ElementsPerPage, Mode=TwoWay, UpdateSourceTrigger=Explicit}" >
. . .
</Custom:MyOwnListBox>

PageDownButton is an IApplicationCommand, ElementsPerPage is a property exposed by the presenter. Now, the first time I open this view the buttons made in that way look enabled but they aren't 'clickable'. If I switch to a different view and I go back to the view with those button, they finally catch the correct behavior. It looks like it doesn't initialize correctly the first time the condition of the command (infact they should be disabled until I insert an item in the listbox), as if the parameter given via the CommandParameter property isn't initiliazed correctly. But I can't understand why switching between the views make it works.

I suspected I should force the UpdateSource of the bindings (I did it for ItemsListBox.NumPageElements and for PageDownButton.CommandParameter) after the view has been loaded, but doing it in the code behind was not of any help.

What I am doing wrong? Thanks for any reply, Mat.